They are firing correctly.
This type of injection system is known as a "batch fire" system.
All the injectors do fire at the same time, unlike a "sequential fire" system where each one fires individually.

All Isuzu engines up to '98 fire this way, if I'm not mistaken.
It wasn't until '98 and up until they went to the sequential fire systems.

If you are having problems setting the timing, you might
want to check that balancer. The outter ring with the timing marks on it are known to slip on the pulley and it will throw your timing marks all out of whack.

1995 Trooper LS auto 3.2 DOHC /w SOHC intake
1989 Trooper 2.6 auto
1989 I-Mark RS DOHC 1.6
1991 Stylus XS DOHC 1.8